This invention is in the field of noninvasive medical monitoring and more specifically in photoplethysmographic monitoring and provides improved measurement accuracy when performing photoplethysmographic measurements on one or more animal species.
In the science of photoplethysmography, light is used to illuminate or trans-illuminate living tissue for the purpose of providing noninvasive measurements of blood analytes or other hemodynamic parameters or tissue properties. In this monitoring modality light is directed into living tissue and a portion of the light which is not absorbed by the tissues, or scattered off in some other direction, is detected a short distance from the entry point. The detected light is converted into an electronic signal that is indicative of the received light signal from the tissue. These signals, one for each emitter, or spectral band of light incident on the tissue-under-test, vary with the pulsation of the blood through the tissue-under-test and are referred to as photoplethysmographic signals. These photoplethysmographic signals are then used to calculate blood analytes such as arterial blood oxygen saturation and hemodynamic variables such as heart rate, cardiac output, or tissue perfusion. Among the blood analytes that may be measured by photoplethysmography are the various types of hemoglobin, including the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood. A device which detects and processes photoplethysmographic signals to measure the levels of various blood analytes and various hemodynamic parameters is referred to as a photoplethysmographic measurement apparatus, device, or instrument.
The first widespread commercial use of a photoplethysmographic measurement apparatus in medicine was in the pulse oximeter, a device designed to measure arterial blood oxygen saturation. To make these measurements at least two different bands of light must be used with each light band possessing a unique spectral content. Each spectral band, or light band, is typically referred to by the center wavelength, or sometimes by the peak wavelength, of the given band. In conventional pulse oximetry two different emitters such as light emitting diodes (LEDs) are commonly used to generate the sensing light. Usually one LED has a center, or peak, wavelength near 660 nanometers (nm) and a second has a center, or peak, wavelength near 900 nm. More recently photoplethysmographic instruments have been developed in which more than two light bands are utilized to allow the measurement of a larger number of blood analytes including carboxyhemoglobin, methemoglobin, and reduced hemoglobin.
Light from each emitter (each light band) is incident on the tissue-under-test, which, for people, usually consists of a finger, earlobe, or other relatively thin tissue site which is well perfused with blood. On non-human species, such as dogs, cats, or other animal species, the tissue-under-test may be the pinna of the ear, the buccal mucosa, the tongue, the web of the toes, or some other acceptable site for transmission or reflectance measurement of photoplethysmographic signals. After passing some distance through the tissue-under-test, a portion of the light not absorbed by the tissue or scattered in some other direction is collected by a detector such as a photodiode and converted into electronic signals that are proportional to the received light signals. The channels, or electronic signals from each of the different light sources, are kept separated or can be separated later through the use of any one of a number of different well-published techniques, including but not limited to, time-division multiplexing or frequency-division multiplexing.
These photoplethysmographic signals received from the tissue consist of a small pulsatile component and a rather large relatively constant component that changes slowly over time when compared with the pulsatile component of the signal. The pulsatile component of the photoplethysmographic signal is created by the pulsation of the blood in the tissue-under-test. When the heart contracts, blood is pushed out of the heart and into the peripheral tissues. This increases the optical density of the tissue located between the emitter and detector elements of the sensor, which decreases the amplitude of the received optical signals. As the heart relaxes and refills with blood the optical density of the tissue-under-test decreases and the received signal amplitude increases. The comparatively constant component of the photoplethysmographic signal is often referred to as the DC component of the signal, and the pulsatile component of the photoplethysmographic signal is often referred to as the AC component of the signal.
The photoplethysmographic signals, after being converted into electronic signals, are processed through the signal processing circuitry to obtain a measurement of the desired blood analytes and, or, hemodynamic parameters which in the case of pulse oximetry typically consists of oxygen saturation in the arterial blood and the heart rate. The signal processing circuitry varies from manufacturer to manufacturer but typically incorporates the steps of amplifying the signals, filtering out unwanted frequencies, and converting the signals into the digital domain. Once in digital form the signals are run through a calibration to determine the desired blood analyte levels. The calibration is any method, means, algorithm, software, equations, or even analog circuitry, which associate the photoplethysmographic signals, with the desired blood analyte level. The steps defined in this paragraph can be carried out by analog circuitry or the photoplethysmographic signals can be converted into the digital domain almost immediately after the photo-detection and these same steps can be performed by manipulation of the digital signals.
In conventional pulse oximetry the calibration is typically an equation (or one of a series of equations selected based on the spectral content of the specific emitters used) which convert the photoplethysmographic signals obtained from the tissue-under-test, after the electronic signal processing, into the measurement of arterial oxygen saturation.
These calibrations or calibration equations are usually derived empirically by performing “desaturation” studies on healthy human volunteers where the photoplethysmographic signals are correlated with invasive measurements of arterial oxygen saturation, or other desired blood analytes of interest. While this work has generated numerous accurate commercial pulse oximeters for use on human subjects, the need to provide species-specific calibrations for photoplethysmographic devices designed for the measurement of blood analytes and or hemodynamic parameters on non-human subjects has remained unmet.
The need for accurate noninvasive measurement of arterial oxygen saturation in veterinary medicine is in some ways even greater than it is for human use. This stems from the fact that there are so few monitoring modalities designed and built specifically for use in veterinary medicine. This can make it extremely difficult to adequately monitor animals during veterinary procedures, thus increasing the morbidity and mortality associated with such procedures.
While some medical monitors designed for use on humans, such as ECG for example, perform accurately on non-human species, this is not the case for photoplethysmographic measurements. Photoplethysmographic measurements of various types of hemoglobin, such as oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin, are dependant on the extinction curves of these various types of hemoglobin. Extinction curves define the absorption of light by a given material as a function of wavelength. The reason one calibration (or one set of calibrations designed to account of the different spectral contents of different emitters) works accurately for all human patients is because the extinction curves of human adult hemoglobin is the same from one person to the next. This is not true across species.
W G Zijlstra et al in 2000 published a book entitled Visible and Near Infrared Absorption Spectra of Human and Animal Haemoglobin, Determination and Application, in which they provided the measurements of the extinctions of the four primary types of hemoglobin for several different species including, human, dog, rat, bovine, pig, horse, and sheep. Analysis of these extinction curves show differences between the species that result in significant measurement errors when a human calibrated pulse oximeter is used on a non-human subject. These errors vary by species and by oxygen saturation level.
This species-to-species variability in pulse oximetry measurements has been demonstrated by N. S. Matthews, et al in their 2003 paper entitled An Evaluation of Pulse Oximeters in Dogs, Cats, and Horses, where they observed that “monitors appeared to perform differently on different species while the techniques and sampling methods were similar.”
Nonetheless all pulse oximeters in veterinary use today contain only one calibration (or set of calibrations for the purpose of accounting for the different spectral contents of specific emitters used in the different sensors) which is used regardless of the species of the patient. These photoplethysmographic instruments contain human calibrations which are typically inaccurate when used on non-human subjects and do not in any way compensate for the differences in optical properties from one species to the next.